News Archive

Most of our effort has been focused on getting the vehicle
all dialed in for the demo flights at the X-Prize cup in a couple weeks.

This particular vehicle is not very well balanced because of
the odd tank shapes and relative sizes of the various components.We have used a combination of a few plates of
ballast weight and gimbal center offsets to get it to
stay centered during the entire flight.The
way the position hold code works, any CG offset or gimbal
misalignment will translate into a fixed position offset that the vehicle will
wind up at when everything settles down.I can always manually steer the vehicle around, but I dont want to rely
on that.With the initial positions and
weights, that center point was a few meters away.

There are a couple little control
system wrinkles that are obvious in retrospect, but took a couple flight tests
to realize.When the flying vehicle is
offset from the start position, the control logic will cause the vehicle to
point back towards where it wants to be.If the vehicle is to the right of the desired spot, the engine plume
will be directed to the left to make the nose rotate back to the left to head
the vehicle back that way.However,
pointing the engine to the left will accelerate the vehicle to the right as it
is rotating the vehicle, so it actually does the opposite of what you want in
the short term.This has to be accounted
for by making sure that the gain from the horizontal velocity is enough smaller
than the gains from attitude and rate that the temporary increase doesnt make
it oscillate badly.This behavior is
much less noticeable on differentially throttled vehicles or attitude-jet
vehicles.

Related to that, dialing in the gimbal
angle so that it is pointing exactly through the cg on the pad will cause the
vehicle to accelerate that direction immediately after liftoff if the CG isnt
straight above the engine, forcing it to swing back to correct.We tried to remove all the ballast weights
and use just gimbal offsets, but that caused enough
of an initial horizontal shove that we wound up leaving 12 pounds of ballast on
one side of the vehicle to let the gimbal stay a bit
straighter.

A testing lesson we have learned is that when we support the
vehicle on stands that are intended to blow out when the vehicle lifts off, you
want the metal sails that catch the wind to be at the bottom of the stands,
rather than the top.Intuitively, you
would think it would be easier to get them to tip over with the sail at the
top, but when the rocket plume hits the ground, all of the gas is directed
horizontally very close to the ground.We had some tests where the stands basically just scooted sideways when
they had a sail on the top, because the wind on the bottom from the deflected
plume was enough to equal the extra leverage that the top sail got.Cutting off the top sails and putting them on
the bottom lets the stands reliably blow out cleanly from underneath elevated
tests.

We knew we were begging for it with this vehicle in open
form, so we werent all that surprised when we finally did catch the tether
ropes on something sticking out of the vehicle.We tipped the vehicle over and bent a couple legs.

We wound up killing two birds with one stone  it had been
decided that it would be more crowd pleasing to fly the vehicle with the
conical aeroshell, so we changed our mounting for the
cone to be robust enough to handle a tether catch on a dropping vehicle, and
tethered it from the top of the cone, as we had done in the big peroxide
vehicles.

Putting the cone on had two unexpected effects.On the positive side, some of the
oscillations that I couldnt seem to make go away just disappeared when the
cone was put on.Russ mentioned that the
oscillations looked like a parachute without a vent hole, so we are theorizing
that the rocket engine plume entrained enough air (you can see the cool mist
around the lox tank being sucked down in some videos) that the flow over the
messy vehicle and flat blast plate was destabilizing.Adding the smooth aeroshell
apparently cleaned that up.

On the downside, we were then getting noisy signals on the
accelerometers when we tried to take off from the ground, evidently from acoustics
inside the cone.We added acoustic foam
around the electronics box and changed the liftoff software a bit to get around
the problem.We know that bad vibration on
the accelerometers can cause problems with the automatic throttle control, so I
have also added some emergency override controls to let me nudge the throttling
down on top of the automatic control if necessary.

A totally unexpected thing we saw was the big effect that
the hot plume had on the steel plate we liftoff from.It was an www.xprizenews.org
forum member that pointed out that heating the center of a plate causes it to
crown up in the middle, which caused our tie-down test chains to go taught last
month, and we saw this clearly on the bigger steel plate with some of our liftoff
tests.The surprising one was when the
vehicle flew a couple feet off to the side and strongly heated the edge of the
plate, causing it to bow corners-up into a half-dish shape.This is still a little worrying, and could
result in a tip-over.We are pretty
confident now that the vehicle is going to stay within a foot of its liftoff
point and land with all four legs on the plate, but if we go off course for
some reason, it might be an issue.

All of the tests have show that bang-bang roll control thrusters
really dont consume much pressurant, even with
helium.We were worried about this, but
it turned out to not be a big deal.

The thermal load on the vehicle base and legs also wasnt as
bad as we expected.We were worried
after melting so many things on the test stand, but 30+ second burns at close
proximity are much worse than the few seconds near the ground at liftoff and
landing that the vehicle sees.

So, we should be good to go.The plan is to try to make three flights in an hour at the X-Prize Cup
event.The first two will be similar to the
hover tests we have been doing, just going somewhat higher, but, pending a
final approval, it looks like we are going to try and make the third flight a
boosted hop to 100 or so.This will
involve a code path and flight conditions that we havent tested on the vehicle
yet, because we didnt want to risk wrecking it before the event.It would be the first thing we would try when
we got back, so if everything gets approved, we might as well try it
there.The crowd may get to witness a
rocket crash

We have done thirteen liftoffs, but these are representative
samples:

Four early open / elevated tests: (13 megs)

First test had a leaking lox fitting
Second test was a 26 second burn-to-depletion with five gallons of ethanol
(half full)
Third test was a short test of adjusted gain to reduce oscillations
Fourth test was a ground liftoff

We have also done a bit of engine development.The regeneratively
cooled throatless engine with film cooling holes ran
for 30 seconds without burning through, but there was some aluminum starting to
come off just above the film cooling holes, so it probably wouldnt have run
for too much longer.We have made a
second one that moves the film cooling holes up another inch, reduces the lox
injector count from 20 to 16 to keep the mixture ratio richer, and I carefully
reduced the runout on the cooling channels a
bit.With 0.030 deep cooling channels,
the runout on my mills rotary chuck makes a
significant difference.We will test
this when we get back from the XPC, and I expect it to work.Unfortunately, it isnt quite big enough to
fly the vehicle, so we will have to hog out bar stock instead of just using the
conveniently sized pipe and tube that we experiment with.

I am drilling the injector pipe for the 5,000 lbf test engine right now, which is the last part to fab for it.This
will be an ablative test engine to just check out the injector behavior, but I
am getting a gun-drilled tube quoted to make a regen
version.We will probably have this
engine with us at the XPC, but we wont fire it until we get back.

We decided to just use a manifold of eight small carbon
fiber pressure bottles for the big vehicle.They are only $475 each, and the packaging will work out great, stacking
them around the edge of the intertank between the two
spheres.We will be initially splitting them
evenly between the lox and ethanol pressurization systems (completely separated
this time to avoid the need for check valves), but they may wind need to add a
fifth one to the lox side because of the extra cooling the pressurant
will receive.

We havent quite decided if we are going to put a seat in
the next vehicle or not.We dont expect
anyone to actually ride in it, but there would be some testing benefit to
having it in a theoretically commercial configuration.The downside would be an extra 4 of length
to the vehicle.